Relaxation models for single helical reversed field pinch plasmas at low aspect ratio

Abstract

A recent study [R. Paccagnella, Phys. Plasmas 23, 092512 (2016)] about the scaling with the aspect ratio (i.e., the ratio between the major and minor radius of the torus) of the dominant mode in single helical reverse field pinch plasmas has shown, at intermediate aspect ratio, that the dominant toroidal mode number in the helical states can be interpreted as the result of a relaxation process. In this work, the same theoretical model is compared and validated with the experimental data obtained in the low aspect ratio RELAX device [S. Masamune et al., J. Phys. Soc. Jpn. 76, 123501 (2007).]. Some difficulties have emerged in this comparison that may be interpreted either as related to a residual level of magnetic chaos in the RELAX plasmas or possibly to genuine toroidal effects not analyzed in this paper, where a cylindrical approximation is considered.